Witnesses can appreciably advance your case while testifying in your opponent's deposition. But the difference between a successful deposition and an unsuccessful one may be whether your witnesses understand how to connect their testimony to your strategy. Counsel should keep in mind the following "best practices" that will focus your attention on the most important elements of deposition witness preparation.
Above all else, counsel must maintain candor toward the court in all judicial processes. Model Rules of Professional Conduct R. 3.3 (1983) ("Candor Toward The Tribunal"). Your witness' testimony must conform to facts that counsel knows to be true and counsel must take remedial action whenever a witness' testimony fails that standard.